Cross Reference to Related Applications
Reference is hereby made to the following copending U.S. Patent Applications filed on even date herewith:
U.S. Ser. No. 409,256, entitled "Image Sensor and Rangefinder Device Having Background Subtraction With Paired Analog Shift Registers" by C. Anagnostopoulos;
U.S. Ser. No. 409,259, entitled "Image Sensor and Rangefinder Device with Background Subtraction Using Interlaced Analog Shift Registers" by D. Lambeth; and
U.S. Ser. No. 409,258, entitled "Rangefinder with Serial Readout Linear Image Sensor and Peak Detector with Threshold Setting Means" by D. Lambeth.
1. Field of the Invention
The present invention relates to automatic rangefinders and image sensors for use therewith, and more particularly to such rangefinders of the type wherein a beam of light is projected on a scene to illuminate a small area on an object, and an image sensor senses the location of the small area in an image of the scene to determine the distance to the object in the scene.
2. Description of the Problem
In one type of automatic rangefinder, a beam of light is projected upon an object in a scene to form an illuminated small area on the object. An image of the scene including the illuminated small area, is formed on a linear image sensor. The location of the illuminated small area along the image sensor is determined by the distance to the object in the scene. Such a rangefinder device finds application for example in an automatic focusing camera.
The general arrangement of elements and mode of operation of such a rangefinding device will be described with reference to FIG. 1. Means for projecting a beam of light is shown by way of example as a light emitting diode (LED) 1 and a lens 2. The beam is projected along a path 3 to illuminate a small area on an object O.sub.1 in a scene. The scene is imaged by a second lens 4 onto an image sensor comprising a linear array of photosensors 5. The signals produced by the photosensors are analyzed by control electronics 6 to determine the position of the illuminated small area in the scene and produce a signal representing the distance to the object.
As shown by example in FIG. 1, the apparent position of the illuminated small area in the scene is a function of distance along light path 3 to the object. For an object O.sub.1 located at a distance D.sub.1 from the rangefinding device, the image of the iluminated small area will fall on the sensor array at location S.sub.1. For an object O.sub.2 at a further distance D.sub.2, the image of the small area will fall on the sensor array at location S.sub.2. By examining the output of the sensor array, the control electronics determines (for example, by comparing the outputs of the elements to determine that output which is a maximum) the location of the illuminated small area in the scene and thereby the distance to the object.
When the ambient illumination is relatively bright, the scene itself, regardless of the illuminated small area, will produce substantial modulation of the photosensor array. Under such circumstances, it is not possible to determine the location of the illuminated small area merely by examining the outputs of the photosensors. The effects of ambient scene light can be removed however by operating the sensor with the beam off and again with the beam on, and subtracting the signals produced thereby. The information in the difference signal is solely from the illuminated small area. One way of accomplishing this subtraction is shown in U.S. Pat. No. 4,274,735 issued June 23, 1981 to Tamura et al. Tamura et al disclose a device having a pair of memory capacitors associated with each photosensitive element in the array. The capacitors are selectively connectable to the photosensitive elements by field effect transistor (FET) switches, and each pair is further connectable to an associated differential amplifier. In operation, a photosensor is first connected to one of the capacitors while the light beam is off. The photosensor is then connected to the other capacitor while the light beam is on. Both the capacitors are then connected to the differential amplifier to produce a photosignal free from background information. The photosignals are compared in an array of comparators to determine the location of the illuminated small area by sensing the largest difference signal from the array of differential amplifiers.
To effectively remove the relatively large background photosignal from the relatively small photosignal due to the illuminated small area, each of the differential amplifiers must have very good common mode rejection. Furthermore, so that the amplifiers themselves do not introduce overwhelming pattern noise into the photosignals, they must be very closely matched. The need for a plurality of closely matched differential amplifiers exacts a high price on any attempt to increase the signal-to-noise ratio of the device.
Furthermore, the use of an array of comparators to determine the maximum difference signal from the differential amplifiers complicates the signal processing circuitry.
There was a need therefore, to simplify the signal processing electronics of the rangefinder while improving the signal-to-noise ratio achievable therein.